[css-position] Added 4 new sticky tests

[TalbotG]

On my website:

http://www.gtalbot.org/BrowserBugsSection/CSS3PositionedLayout/sticky/position-sticky-stacking-context-002.html

http://www.gtalbot.org/BrowserBugsSection/CSS3PositionedLayout/sticky/position-sticky-scrolled-remove-horiz-sibling.html

http://www.gtalbot.org/BrowserBugsSection/CSS3PositionedLayout/sticky/position-sticky-fixed-ancestor-002.html

http://www.gtalbot.org/BrowserBugsSection/CSS3PositionedLayout/sticky/position-sticky-fixed-ancestor-003.html

http://www.gtalbot.org/BrowserBugsSection/CSS3PositionedLayout/sticky/reference/position-sticky-fixed-ancestor-002-ref.html

[fantasai]

• position-sticky-stacking-context-002.html - I think this is a good test, but it would be better if we had another div to check the overlap on top. One way to do this would be to make the sticky item half green and half red, and to make the third div a half-sized div that overlaps the red half.

• position-sticky-scrolled-remove-horiz-sibling.html - Instead of adding a new file, please incorporate this test into the original file.

• position-sticky-fixed-ancestor-002/3.html - I believe these tests are correct in that they should pass as described... but I don't understand what the assert is trying to say. Also, are there equivalent tests for other values of 'position', given they should all behave the same here?

[fantasai]

(See above)

[TalbotG]

• position-sticky-stacking-context-002.html - I think this is a good test, but it would be better if we had another div to check the overlap on top. One way to do this would be to make the sticky item half green and half red, and to make the third div a half-sized div that overlaps the red half.

Understood. On my to-do-list now.

• position-sticky-scrolled-remove-horiz-sibling.html - Instead
  of adding a new file, please incorporate this test into the original
  file.

Understood. On my to-do-list now.

• position-sticky-fixed-ancestor-002/3.html - I believe
  these tests are correct in that they should pass as described...
  but I don't understand what the assert is trying to say. Also,
  are there equivalent tests for other values of 'position', given
  they should all behave the same here?

position-sticky-fixed-ancestor-002.html (3 selectors and 10 declarations) is a simplified version of
http://wpt.live/css/css-position/sticky/position-sticky-fixed-ancestor.html (8 selectors and 26 declarations)

position-sticky-fixed-ancestor-003.html (3 selectors and 10 declarations) is a variation of
http://wpt.live/css/css-position/sticky/position-sticky-fixed-ancestor.html

I will improve the assert text, otherwise remove the current one. Right now, removal of current assert text is my preference.

Right now, there are no additional tests for other values of 'position' for (or with) this kind of code scenario.

[TalbotG]

Both
position-sticky-scrolled-remove-horiz-sibling.html
and
position-sticky-scrolled-remove-sibling.html
have been merged into 1 single test. At my website:

http://www.gtalbot.org/BrowserBugsSection/CSS3PositionedLayout/sticky/position-sticky-scrolled-remove-sibling-new.html

I have no idea why the nested
requestAnimationFrame(()=>{...}
are important or required and what
they are supposed to do. I will add them back
if requested. Adding @mstensho as he
would certainly know.

[TalbotG]

At my website, the improved
position-sticky-stacking-context-002
test is:

http://www.gtalbot.org/BrowserBugsSection/CSS3PositionedLayout/sticky/position-sticky-stacking-context-002-new.html

[mstensho]

I have no idea why the nested requestAnimationFrame(()=>{...} are important or required and what they are supposed to do. I will add them back if requested. Adding @mstensho as he would certainly know.

I don't really know (I've just accepted to (almost) always use double RAF in tests that require the page to be painted before making dynamic changes). Maybe see https://bugs.chromium.org/p/chromium/issues/detail?id=675795 ?

[TalbotG]

http://www.gtalbot.org/BrowserBugsSection/CSS3PositionedLayout/sticky/position-sticky-stacking-context-002-newer.html

includes adjustments according to review comments.